Nested If formula

Hi everyone!

I have a formula for my excel workbook. The formula for one cell contains multiple arguments and goes like this:

If B13="", then "", else

IF(OR(AND(D8>=1,D8<=5),AND(D8>=16,D8<=25),AND(D8>=36,D8<=45)),"C1",
IF(OR(AND(D8>=6,D8<=15),AND(D8>=26,D8<=35),AND(D8>=46,D8<=50)),"C2",
IF(OR(AND(D8>=51,D8<=55),AND(D8>=66,D8<=75),AND(D8>=86,D8<=95)),"C3",
IF(OR(AND(D8>=56,D8<=65),AND(D8>=76,D8<=85),AND(D8>=96,D8<=100)),"C4","")))).

I was able to apply this formula to one cell but what happens is, If I drag the cell with the formula to the next cell, the referenced cells changed. What I'm hoping to achieve is that only the range from B13 downward will change and the rest of the formula stays the same. Can anybody help me on this? Thank you!

Thank you for your suggestion. I was able to get the result I needed.